1. Field of the Invention
The present invention relates to the yield of switching regulators.
2. Prior Art
Voltage regulators of various types are well known in the prior art. Of particular importance to the present invention are switching regulators which allow the regulators to provide a regulator output of a voltage which can be either higher or lower than the input voltage, as desired or as required for a particular application. One application for such regulators is with respect to systems operating on rechargeable batteries such as, by way of example, lap top computers. In this application, a charged battery may provide an input voltage to the regulator exceeding the desired regulated output voltage thereof, though with the battery voltage sagging to a voltage below the desired output voltage of the regulator as the battery discharges. The time between recharges necessary in such an application may be extended if the system will operate on a significantly reduced battery output voltage until such time as the battery is nearly fully discharged. The time between recharges can also be extended if the efficiency of the regulator in such a system is improved and, of course, the size of various components of the regulator, such as the inductor, can be decreased with such increased efficiency because of the proportionately lower power dissipation in the regulator. Similarly, battery powered systems such as laptop computers may be operable with different types of batteries or battery packs having different output voltages, some requiring step-up and some step-down in the battery voltage to provide the desired regulated voltage for system operation. Also such systems are often operable on a battery pack or an AC-to-DC converter, the two frequently having significantly different voltages to present to the regulator. These are merely exemplary of the many applications in which this type of regulator is useful, if not mandatory.
Prior art switching regulators having the capability of both step-up and step-down are SEPIC (single-ended primary inductance converter), Flyback, Step-up plus LDO (low dropout linear regulator), or are sometimes comprised of the basic circuit shown in FIG. 1. As shown therein, an inductor L is coupled to the input voltage V.sub.in through a p-type MOS transistor PFET, and is coupled to the output voltage through diode D2. Capacitors C1 and C2 provide smoothing on the input V.sub.in and the output V.sub.out, respectively, with NMOS transistor NFET controllably coupling node LX2 to ground. In operation, transistors PFET and NFET act as switches, with the controller controlling the switches turning both transistors on or both transistors off at the same time. In particular, when both transistors PFET and NFET are turned on, current in the inductor L will build up at the rate ##EQU1## During this time, diode D1 will be back biased, as will diode D2 if there is any output voltage V.sub.out, or at least on initial start-up will not be forward biased. Thus during this time, current flow is through the PFET transistor, inductor L and the NFET transistor to ground, with some i.sup.2 R loss occurring in the inductor L and transistors even though no current is being delivered to the output of the regulator. When both transistors PFET and NFET are turned off, the magnetic field of the inductor L decays, forward biasing diodes D1 and D2 and providing a current to the regulator output initially equal to the current through the inductor when the transistors were first turned off, and decreasing at the rate: ##EQU2## where V.sub.D is the voltage drop of each diodes D1 and D2 when forward biased to conduction.
To minimize the forward conduction diode voltage drops of diodes D1 and D2, Schottky diodes may be used.
It may be seen from the foregoing description that current is flowing in the inductor L in both modes of operation, even though the current in the inductor is only flowing to the output of the regulator during the second mode of operation. Thus, there is a significant i.sup.2 R loss in the inductor during both modes of operation, even though current is delivered to the output only during the second mode. By way of a specific example, if the regulator is operated so that the output voltage equals the input voltage minus 2V.sub.D, the current through the inductor L will have a triangular waveform, with as much i.sup.2 R loss in the rising part of the waveform as in the decreasing part of the waveform, even though current is delivered to the output only during that decreasing part. This power dissipation can be kept down by making the inductor and transistors physically larger, though this is undesirable from a cost standpoint, and not consistent with the trend toward further miniaturization and complexity of electronic systems.
Due in part to complexity and efficiency issues, prior art step-up/step-down regulators of the foregoing type have not yet been implemented in monolithic integrated circuit form, but have been produced using discrete circuits.